Radiant energy burners employ a combustion element which is permeable to the gaseous fuel and the fuel is burned in a flameless type of combustion on the outer surface of the element to principally emit radiant energy. In burners of this type, it is important to control the porosity and back pressure of the combustion element in order to obtain the proper combustion efficiency and minimize the possibility of "blowback" or flame lifting from the surfce of the burner.
In the past, a form of radiant combustion element has consisted of an inner metal screen covered with a layer of randomly disposed short ceramic fibers. Elements of this type have been produced by immersing the screen in a molding tank containing a liquid slurry of the ceramic fibers and then drawing a vacuum through the screen, with the result that the fibers are deposited as a layer on the screen. The resulting vacum-formed layer of ceramic fibers is fragile and is highly susceptible to damage during shipment and handling.
During use, the short fibers in the vacuum formed layer tend, with time, to dissociate which results in the combustion element having a non-uniform porosity, thereby decreasing the efficiency of the combustion and the useful life of the burner.
Furthermore, if the vacuum formed fibrous coating is broken away, either by damage or during usage, an outage can result in which a flame sensor will shut down the system due to a significant change in combustion pattern. While an outage is not a dangerous situation, it is a nuisance problem.
To provide protection for the fragile vacuum formed coating, attempts have been made in the past to enclose the combustion element in an outer protective sleeve, such as described in U.S. Pat. No. 3,275,497 and 3,179,156. However, the use of an outer protective screen substantially reduces the efficiency of the radiant heating operation and adds unnecessary cost.